Sunday, October 05, 2008

More belt drive thoughts

A fun weekend is almost behind us - I am finally not sick, and (kinda) not broken, so I was actually able to bandage up my bum wrist and go for a road ride both Saturday and Sunday. Sweet! It's not improving as quickly as I'd hoped, so I think mountain biking is done for the year, but if I can get out on the road at least, I can keep my sanity.

Spent some time this morning playing with belt drive parts and plotting. After looking at the info from Spot, and mounting up the drive cog/chainring on an LX crankset (ie, 50mm chainline) to do some real-world visualization, it appears to me that I can, with some tubing manipulation and trickery, manage to build a singlespeed 29er with 445mm (or about 17.5") chainstays, clearance (just barely) for a 2.1" tire, and clearance (again, by the skin of my teeth) for the chainring thing. It's not anything like what's doable with a chain, of course (you could probably get the stays down into the 425mm range pretty easily) but I'm a big guy and really prefer the stays in the 440-450mm range as it is.

This is a good sign, because last I looked at it (without actually mounting anything up) it really appeared to me that there was no practical way to get any kind of mountain bike tire onto a belt driven bike without making the chainstays super duper long (this is the solution that Spot seems to be using - 18.5"+ chainstays) or doing something really weird with elevated chainstays (barf).

I'm cautiously optimistic that it's doable - but it'll require several tweaks:
-2mm spacers between the crank spider and the chainring/cog to offset the beltline outboard and make extra room. Yes, this is a little bit sketchy as it puts all the force on the bolts instead directly onto the spider, but I think it'll be fine.
-Fairly significantly crimp some S-bend stays for tire clearance (this will get me another 2mm or so on each side of the tire). I do this anyway on a lot of bikes, but never bothered with my current singlespeed, because I don't personally want super short stays or huge rear tires.
-Narrow the tire clearance (at the widest point on the knobs) to somewhere around 60mm. This of course leaves very limited room for anything bigger than a ~2.1" tire (no Weirwolves on the rear of this bad boy) but since I hate using big tires on the rear anyway, it shouldn't be much of an issue. I just better hope there's no mud...

In any case, it's looking like I might actually start this project in the next few days, so stay tuned if you're interested in this sort of thing. I'm also attempting to do a seat tube that will take a 30.9 (Speedball!) seatpost, as well as a few other fancy bits. My loss (wrist injury) is your gain (more framebuilding posts), loyal blog readers!

And yes, I'm aware that this is several consecutive posts with no pictures. Sorry. I'll try to do better.


steve garro said...

dude! heal up/get well! and, good luck with that beltdrive thingy.....steve.

John Caletti said...

Hey Walt,
I recently did a belt drive 29er and used Columbus S bend stays - squeezed a bit for tire clearance (fits hutch pythons well). Chainstay length was supposed to be 440 with the gear combo I used, but actually comes out a little longer - the CDS chart/spec is a little off it seems. I used an XT crankset and put a very small dimple on the outside for a bit more room for the cog. Something to be aware of, is that the rear cog needs to use only one small spacer between it and the lockring, if you space out the front ring away from the frame the belt will want to walk off to the inside and could come off. I found this on my test ride when I didn't have the rear spacers set up right. Call me to talk more. I'm interested in other dropout options - like modifying a Paragon slider. Good Luck, John Caletti